PEPD and Alzheimer disease: Most naturally occurring human forms of PrD represent a group of extremely rare (incidence 1–10 per 100 million) fatal, progressive, transmissible neurodegenerative encephalopathies that appear to arise following the misfolding of the cellular prion protein (PrPC) into an immunogenic disease-associated conformation (PrPSc), or for related lethal neurodegenerations such as AD and accumulation of extremely insoluble, highly pro-inflammatory Aβ42 peptide-containing amyloids.